1. Field of the Invention
The present invention relates to a boundary acoustic wave device using a boundary acoustic wave propagating through the interface between media and a method for producing the boundary acoustic wave device. More specifically, the present invention relates to a boundary acoustic wave device including electrodes disposed between a first medium and a second medium, and another medium laminated on the outer surface of one of the first and second media, and also relates to a method for producing the boundary acoustic wave device.
2. Description of the Related Art
Various surface acoustic wave devices have been used for RF and IF filters in mobile phones, resonators in VCOs, and VIF filters in television sets. Each of the surface acoustic wave devices use a surface acoustic wave, such as a Rayleigh wave or a first-order leaky wave, propagating along a surface of a medium.
Surface acoustic waves propagate along the surface of a medium, and are thus sensitive to changes in the surface condition of the medium. Accordingly, to protect the surface of the medium along which the surface acoustic waves propagate, in the related art, the surface acoustic wave element is hermetically sealed in a package having a cavity such that the surface of the medium described above is disposed therein. The use of the package having the cavity inevitably increases the cost of the surface acoustic wave device. In addition, the package has significantly larger dimensions than the surface acoustic wave element. Thus, the size of the surface acoustic wave device is relatively large.
Acoustic waves other than the surface acoustic waves include boundary acoustic waves propagating along the boundaries between solids.
For example, “Piezoelectric Acoustic Boundary Waves Propagating Along the Interface Between SiO2 and LiTaO3” IEEE Trans. Sonics and ultrason., Vol. SU-25, No. 6, 1978 IEEE (Non-Patent Document 1) discloses a boundary acoustic wave device including an IDT disposed on a 126°-rotated Y-plate X-propagating LiTaO3 substrate and a SiO2 film having a predetermined thickness disposed over the IDT and the LiTaO3 substrate. This document describes that SV+P mode boundary acoustic waves called Stoneley waves propagate. Non-Patent Document 1 also describes that when the SiO2 film has a thickness of 1.0λ (wherein λ represents the wavelength of the boundary acoustic wave), the electromechanical coupling coefficient is 2%.
Boundary acoustic waves propagate with their energy concentrated on the boundaries between the solids. Thus, minimal energy is present on the bottom surface of the LiTaO3 substrate and on a surface of the SiO2 film. Therefore, characteristics of the boundary acoustic wave do not vary with changes in the surface state of the substrate or the thin film. Thus, a package having a cavity is not required, thereby reducing the size of the acoustic wave device.
To suppress non-uniformities in resonant frequencies and center frequencies in filters and resonators using the acoustic waves, various methods for adjusting frequencies have been developed. For example, Japanese Unexamined Patent Application Publication No. 5-191193 (Patent Document 1) discloses, in a piezoelectric ceramic filter using the thickness vibration of a bulk wave, a method for adjusting a frequency by evaporating an insulating material onto resonant electrodes disposed on a surface of a piezoelectric ceramic substrate.
Japanese Unexamined Patent Application Publication No. 2-301210 (Patent Document 2) discloses a surface acoustic wave device using a surface acoustic wave, the surface acoustic wave device including a SiN film covering interdigital electrodes and reflectors disposed on a piezoelectric substrate. A center frequency and a resonant frequency are adjusted by controlling the thickness of the SiN film.
WO98/51011 (Patent Document 3) discloses a boundary acoustic wave device shown in FIG. 12. A boundary acoustic wave device 100 includes interdigital electrodes 102 and 102 disposed on a first piezoelectric substrate 101, a dielectric film 103 disposed over the interdigital electrodes 102, and a Si-based second substrate 104 laminated on the upper surface of the dielectric film 103. In the boundary acoustic wave device 100, the Si-based second substrate 104 is disposed on the interdigital electrodes 102 with the dielectric film 103 provided therebetween, and thus is not in direct contact with the interdigital electrodes 102. Therefore, it is possible to reduce the parasitic resistance between the interdigital electrodes 102.
The above-described boundary acoustic wave device does not require a package having a cavity, thereby reducing the size of an acoustic wave device. However, according to experiments conducted by the inventors, production tolerance often induces non-uniformities in a resonant frequency and a center frequency in the boundary acoustic wave devices, as in the case of the surface acoustic wave devices. In particular, in the boundary acoustic wave device, after the formation of the electrodes on a first medium, a second medium was formed so as to cover the electrodes. Thus, if there was production tolerance in the second medium, the frequency of the boundary acoustic wave device would have been likely to vary significantly.
On the other hand, in methods described in Patent Documents 1 and 2, an insulating material is deposited on a surface of a bulk wave substrate by evaporation to adjust the frequency of a bulk wave device. In a method described in Patent Document 2, a SiN film is provided on a surface acoustic wave substrate to adjust the frequency. That is, in a known bulk wave device and surface acoustic wave device, an insulating material or a metal is deposited on a surface of a substrate to adjust a frequency by using the distribution of oscillation energy to the surface of the substrate. Furthermore, in another method, an electrode disposed on a surface of a substrate is etched or a surface of a substrate is etched to adjust a frequency.
However, since the oscillation energy of a boundary wave is scarcely distributed in a boundary acoustic wave device, such a method for adjusting the frequency cannot be used. In other words, if foreign matter such as an insulating material is deposited on a surface of a substrate or if a surface of a substrate is shaved, the resonant frequency and a pass band are not changed.
In the boundary acoustic wave device 100 described in Patent Document 3, interposing the dielectric film 103 reduces parasitic resistance. However, the frequency cannot be adjusted after completion.